1. Field of the Invention
The present invention relates to the art of flash butt welding, and is specifically concerned with a method of flash butt welding in fabricating structures from ferrous-metal rolled products, such as pipes, sheets, plates, T-sections, etc.
2. Description of the Prior Art
It is well known that the flash butt welding process is accompanied by a highly concentrated local heating of the metal through fusing a surface layer thereof by an electric current flowing via the work pieces being welded together. This is followed by the upsetting which levels off the surfaces or faces being joined, forces the molten metal and oxides (if occur) out of the gap between the faces, and thus forms a welded joint.
The formation in the flash butt welding of sound, defect-free joints offering high strength and ductility is ensured by preventing the metal from oxidation or, if this fails to be attained, by removing the oxides jointly with the molten metal from the butt joint through upsetting.
In the course of the fusion, the work pieces being joined are caused to approach each other with the welding current switched on. At the moment when the faces of the work pieces touch each other, electrical contacts form therebetween. At the initial stage of the fusion, the contacts develop between the faces of a solid metal, and later there may appear contacts between the films of a liquid metal or of an oxide thereof.
During the fusion, the work pieces being joined are pressed against each other with a relatively low axial force; consequently, the electrical resistance of the contacts formed in the course of the fusion is high, and as a result a large amount of heat is generated therein. The metal at the contact point and in the region adjacent thereto melts down very rapidly with the formation of liquid metal bridges between the faces of the work pieces being joined, which bridges serve as short-time current conductors.
Moving the faces of the work pieces being joined towards each other brings about an increase in the bridge diameter. At the next moment, however, the current through the bridge increases rapidly, and the effect of the electromagnetic forces causes the bridge diameter to decrease, in which the current density increases, which results in a vigorous heating of the bridge. When the current density reaches a certain magnitude, the metal at the centre of the bridge evaporates. An impetuous rise of the pressure of the metal vapour and gases inside the bridge results in its explosion-like rupture.
The explosion of the molten metal bridge leaves craters on the faces of the elements being joined. The next bridge contact develops, as a rule, at some other place, where the distance between the faces of the elements being joined is minimum. At the point where a molten metal bridge exists at a given moment, the metal is molten, while the metal at the bottom of an adjacent crater may, due to the heat transfer, cool down below the solidus temperature with the formation of oxides.
The fusion of the end faces of the elements being joined prepares them for the formation of the joint, the formation being accomplished by the upsetting. If a layer of a molten metal completely covers the end faces at the moment when these come into contact, a joined interlayer of the metal melt forms therebetween, and the subsequent upsetting progressively closes the gap over the entire area of the joint. At the points of deep craters, where opposite deepest craters coincide with each other, the gap formed therebetween is the largest one. If at the beginning of the upsetting or at the moment when the end faces come into contact there is no liquid metal at such points on the end faces, the joint is formed in a solid phase. Closing all the gaps between the end faces of the elements being joined calls for increasing the amount of upsetting. Both an insufficient and an excessive upsetting are believed to impair the quality of the joint. A small upsetting fails to completely remove the possible oxides from the butt joint and may leave the craters unclosed, whereas the excessive upsetting leads to a sharp distortion of the rolling streaks on the rolled metal.
The practice has shown that the weldments produced by the prior art methods of flash butt welding (Gel'man A. S. Osnovy svarki davleniem (Fundamentals of pressure welding). "Mashinostroenie," Moskow, 1970, p. 159), when subjected to the bending test, feature the formation of first cracks already at small angles of bending of a specimen from its initial position. The first cracks develop at the points where the distorted rolling streaks emerge to the surface of the joint. The practice has also shown that the welded joints of rolled products display a higher ductility if the angle of distortion of the rolling streaks with respect to the longitudinal axis of the elements being welded together does not exceed a critical angle whose value is within 25.degree.-30.degree.. Such a distortion of the rolling streaks in welded joints can be attained at an exactly defined amount of upsetting. This means that the amount of upsetting must be, on the one hand, sufficient enough to close all the gaps in the butt joint and, on the other hand, such as to result in the least distortion of the rolling streaks.
For such an exact determination of the amount of upsetting however, the amount of the maximum gap, .DELTA..sub.g.max, must be found beforehand.
There is also known a method of flash butt welding, disclosed in the article "Selecting the optimum upsetting in flash butt welding of steel pipes" by Livshic V. S. et al. ("Stroitel' stvo truboprovodov" ("Pipeline construction"), 1966, No. 9, p. 18). The method consists in fusing the end faces of rolled work pieces in particular pipes, at a preset no-load voltage and upsetting them. The amount of upsetting is determined with regard to the "temperature at the butt joint boundary," which term denotes the temperature at the pipe cross-section, spaced from the end faces being fused before the upsetting at a distance equal to half the amount of upsetting. Assumed as the temperature of the butt joint boundary in the prior art method is preferably that within from 1,100.degree. to 1,150.degree. C. Determining the amount of upsetting from the temperature of the butt joint boundary shows that to produce a good quality joint of 20-mm thick work pieces being welded together, the amount of upsetting is to be of 14 mm. With such upsetting, as is claimed in the article, the average bending angle is to 100.degree.. The average bending angle is determined by conducting the bending test of 12 specimens cut out at different places of the weld joint. A joint where the average bending angle is not less than 100.degree. is generally assumed as a good-quality joint.
It has been ascertained by experiments that a good quality welded joint, in terms of an average bending angle, is attained with a temperature at the butt joint boundary, ranging from 1,100.degree. to 1,150.degree. C. If the temperature of the butt joint boundary is assumed to be less than 1,100.degree. C., in which case the determined amount of upsetting exceeds the optimum one, or to be greater than 1,150.degree. C., which yields the amount of upsetting below the optimum value, the quality of welded joints is impaired. The first condition (t.degree.&lt;1,100.degree. C.) is essentially correct, whereas the second one (t.degree.&gt;1,150.degree. C.) is wrong inasmuch as a welded joint or a metallic bond during the upsetting arises already at the moment when the elements being welded together come in contact with each other. The metallic bonds in the butt joint may loosen in essentially two events: when the end faces of the elements being joined are before the upsetting covered by a large amount of thick oxide film due to violation of the proper welding conditions and also when the end faces being joined have been excessively heated, which results in a burning of the metal on the faces. The welding conditions being correctly selected, the metallic bond between the elements arises at the moment when these come in contact with each other only in the course of the upsetting.
According to another available data, the upset allowance for work pieces of the same thickness is selected to be of 13 mm, which is close to the amount of upsetting, determined by the above-described method.
With such amounts of upsetting, however, the rolling streaks get considerably distorted in the welding zone. The bending angle of most of specimens of such steel grades as CT. 3, steel 20, 17.GAMMA.1C, 18.GAMMA.2A , 09.GAMMA.2 and X60 subjected to the bending test, is much less than 100.degree., and of some specimens even less than 40.degree..
The welded joints of rolled products are known to display a better ductility if the angle of distortion of the rolling streaks with respect to the longitudinal axis of the elements being welded together does not exceed a critical angle whose value is within 25.degree.-30.degree.. Such a distortion of the rolling streaks in welded joints is obtainable with the minimum possible amounts of upsetting.